A video sequence consists of a number of still images called frames. Coding of a video sequence, video coding, is done by describing the frames as bit-efficiently as possible. To do this, redundancy in the video sequence is exploited. There are three types of redundancies that can be exploited, temporal redundancy, spatial redundancy and spectral redundancy. Temporal redundancy is the redundancy between two frames, while spatial redundancy is the redundancy within a frame. Spectral redundancy is the redundancy between different colour components in the video. In the following we will not consider the spectral redundancy.
Video coding standards define a number of frame types, out of which the I-frame and the P-frame are common to most standards. The I-frame is coded by exploiting spatial redundancy solely, resulting in a representation that is independent of all other frames. P-frames, on the other hand, are coded by exploiting both temporal and spatial redundancies. This leads to a more compact representation of the frame, while at the same time making this representation dependent of an other frame (in most cases the previous one).
Video coding standards from about 1995, e.g., H.263 and later MPEG-2, have been developed for the purpose of bit-efficient video coding and make use of the I-frame/P-frame setup. The applications have mainly included videoconferencing and videotelephony over circuit-switched networks, but also storing video material for later retrieval, e.g., the DVD. Newer standards, e.g., MPEG-4 and H.264, have a performance that is significantly improved over their predecessors and achieve low bit-rates for given video quality. The main ideas of using different frame types have been preserved and the performance improvement is a result of refinement of the methods used in older standards. One such refinement is that a frame can be segmented into smaller regions called slices, and the method of using I frames and P frames can be applied on individual slices.
With the arrival of new technology, where greater processing power and packet-switched networks (WLAN and Internet) have had the leading role, new applications have become of interest. These applications include streaming video and live video communication over IP networks. The requirements that live video communication applications pose on the underlying technique are quite different from those of storage applications and even streaming applications. In addition to the requirements that are present in storage and streaming applications, live video communication poses a strict requirement on the delay between sending and displaying video. This strict requirement makes the overall number of errors in transmission increase, since delayed packets are handled equivalently to lost packets.
Existing video coding techniques, using the mentioned setup with different frame types, are not suitable for live video communication due to the strict delay restriction. Introducing high dependency between frames to achieve a bit-efficient representation of the signal results in display of erroneous video in environments where the probability of transmission error is significant. Not only is it impossible to render frames that are not received in time, but the frame dependency makes the error propagate throughout the video sequence, which is annoying to the viewer. The problem is in current standards handled, in more or less efficient ways, by sending a frame that is independent of other frames, an I-frame. In this manner, the propagated error is reset to zero. However, the choice of how frequently I-frames should be sent is not trivial. Increasing the I frame frequency results in better video quality when there is a possibility of transmission errors, while at the same time increasing the bit-rate. Hence, there is a trade-off between video quality and bit-efficient representation of the video.
Therefore, it is desirable to be able to increase the video quality without having to increase the bit-rate too much, thereby still providing a bit-efficient representation of the video.